I've looked a bit in firefox sources in netwerk/protocol/data/nsDataHandler.cpp
data handler only parses content/type and charset, and looks if there is ";base64"
in the string
the rfc specifices no filename and at least firefox handles no filename for it,
the code generates a random name plus ".part"
I've also checked firefox log
[b2e140]: DOCSHELL 6e5ae00 InternalLoad data:application/octet-stream;base64,SGVsbG8=
[b2e140]: Found extension '' (filename is '', handling attachment: 0)
[b2e140]: HelperAppService::DoContent: mime 'application/octet-stream', extension ''
[b2e140]: Getting mimeinfo from type 'application/octet-stream' ext ''
[b2e140]: Extension lookup on '' found: 0x0
[b2e140]: Ext. lookup for '' found 0x0
[b2e140]: OS gave back 0x43609a0 - found: 0
[b2e140]: Searched extras (by type), rv 0x80004005
[b2e140]: MIME Info Summary: Type 'application/octet-stream', Primary Ext ''
[b2e140]: Type/Ext lookup found 0x43609a0
interesting files if you want to look at mozilla sources:
data uri handler: netwerk/protocol/data/nsDataHandler.cpp
where mozilla decides the filename: uriloader/exthandler/nsExternalHelperAppService.cpp
InternalLoad string in the log: docshell/base/nsDocShell.cpp
I think you can stop searching a solution for now, because I suspect there is none :)
as noticed in this thread html5 has
download attribute, it works also on firefox 20 http://www.whatwg.org/specs/web-apps/current-work/multipage/links.html#attr-hyperlink-download